This invention relates to thermoplastic elastomer compositions including blends of propylene and ethylene polymers for extrusion, calendering, blow molding, thermoforming, and foam processing, and articles made therefrom.
There is a need for recyclable materials that can be used as alternatives to polyvinyl chloride for the fabrication of articles. Polyvinyl chloride, often used with a plasticizer, can be formed into a rubbery, thin sheet for use as a skin layer over a rigid or soft substrate. Due to the combination of the tactile feel (softness) and the melt strength during processing, plasticized polyvinyl chloride can be a very desirable material. Polyvinyl chloride, however, is not easily recyclable or melt blendable with non-polar polymers, which has limited the utility of polyvinyl chloride to applications where recyclability is not desired. Recyclable materials with processing characteristics similar to polyvinyl chloride, such as high melt strength, are being actively sought.
Olefinic polymers, as a class of materials, offer the capability to be recycled with very little loss of physical properties due to the high level of hydrocarbon saturation. In order to achieve a soft tactile feel similar to cured animal leather or polyvinyl chloride sheets in a recyclable product, several thermoplastic polyolefin technologies have been developed.
Olefinic thermoplastic elastomers including thermoplastic olefin blends (TPO), thermoplastic polymer alloy compositions, and dynamically vulcanized thermoplastic elastomers have been explored for such applications.
A thermoplastic elastomer (TPE) is a material that exhibits rubber-like characteristics, yet may be melt processed with most thermoplastic processing equipment, such as by extrusion. The rubber-like characteristics typically desired are high extensibility, mechanical recovery, resiliency, and low temperature ductility. An olefinic thermoplastic elastomer includes primarily polymers manufactured by the polymerization of at least 50 mole percent olefinic monomers, such as ethylene, propylene, butylene, iso-butylene, alpha-olefins, olefinic dienes, and the like.
Physical blends of thermoplastic polyolefins are commercially available as recyclable alternatives to plasticized polyvinyl chloride. One such material, DEXFLEX(copyright) E280, commercially available for thin sheet extrusion from Solvay Engineered Polymers of Auburn Hills, Mich., is prepared by melt blending polypropylene with high molecular weight ethylene-propylene rubbers. This and other similar materials are often referred to as flexible thermoplastic olefins (f-TPO). The advantages relative to polyvinyl chloride are low temperature ductility, weatherability, higher temperature service, and comparable cost per volume. The family of most melt-blended f-TPO products, however, tends to have a lower melt strength for high temperature processing, e.g., high speed sheet extrusion, calendering, thermoforming, blow molding, and foaming.
A polymer blend that includes an irradiated partially crystalline polyolefin with high melt strength and a non-irradiated polyolefin is disclosed in U.S. Pat. No. 5,508,318. This composition exhibits many desirable characteristics for extruded thin sheets, but has the disadvantage of higher cost due to the electron beam irradiation process and the subsequent number of melt blending steps required to achieve the desired material by incorporation of other raw materials and ingredients.
One family of thermoplastic polymer alloy compositions can be prepared from blends of polypropylene, ethylene copolymer ionomer resin, ethylene glycidyl acrylate or methacrylate copolymer, and uncrosslinked ethylene propylene rubber, such as are disclosed in U.S. Pat. No. 5,206,294. The reaction of the epoxide group with the acrylic acid group creates a partially crosslinked network that results in a material with improved melt strength and desirable physical properties. A product similar to this is available commercially as DEXFLEX(copyright) E250 from Solvay Engineered Polymers of Auburn Hills, Mich. This technology tends to be more expensive due to the specialty ethylene-based copolymers that are produced with a high pressure reaction process. Also, these materials tend to exhibit an undesirable high surface gloss when extruded in sheets, which gloss requires additional processing to be removed.
Thermoplastic elastomers called dynamically vulcanized alloys (DVAs) can be prepared through the process of dynamic vulcanization, such as that described in U.S. Pat. Nos. 3,758,643 and 3,806,558. Using this process, an elastomer can be crosslinked during melt mixing with a rigid thermoplastic polyolefin to yield a material that is melt processable, yet exhibits characteristics similar to thermoset elastomers. Compositions obtained with this process are micro-gel dispersions of cured elastomer in an uncured matrix of thermoplastic polymer. Commercial olefinic thermoplastic elastomer materials that use this technology of dynamic vulcanization are well known and are disclosed in U.S. Pat. Nos. 4,130,535 and 4,311,628. The materials disclosed in these patents are commercially known as SANTOPRENE(copyright) and utilize a phenolic resin to crosslink the olefin elastomer phase. The SANTOPRENE(copyright) materials are melt processable and can be extruded into profiles such as sheets. They also tend to exhibit high melt strength, but have very little ductility and draw, which reduces the utility of the material technology for processing applications such as thermoforming, blow molding, and foaming.
The use of organic peroxide to crosslink the elastomer phase in an olefinic-based DVA is well known to those of ordinary skill in the art. For example, U.S. Pat. No. 3,758,643 discloses that peroxide 2,5-bis(t-butylperoxy)-2,5-dimethylhexane at a concentration of 0.05 to 0.4 weight percent is useful for crosslinking the elastomer phase in the olefinic DVA. The use of peroxide alone, however, can be detrimental to the high molecular weight polypropylene due to the beta-scission that occurs and results in a very low molecular weight for the thermoplastic phase. The consequences of this degradation include lower melt strength and poor solid-state mechanical properties.
U.S. Pat. No. 4,454,092 discloses a process for the single-step manufacture of an olefinic-based DVA in which the elastomer is crosslinked with organic peroxide at a concentration of 0.3 weight percent. To minimize the adverse consequences of organic peroxide upon the thermoplastic polypropylene, the free radical crosslinking aid, divinyl benzene, is used as a co-agent at a concentration of 0.5 weight percent. The relatively high organic peroxide content disclosed here tends to cause significant chain scission of the polypropylene, thereby leading to lower viscosity (or higher melt flow rate) and a resulting loss in melt strength properties.
International Patent application No. WO 98/32795 discloses that a thermoplastic elastomer can be prepared from a blend of ethylene-octene elastomer and polypropylene when rheologically modified with organic peroxide at a concentration of 0.15 to 1 weight percent. These materials exhibit improved melt strength and contain less than 10 weight percent of non-extractable gel content as measured with a 12-hour boiling reflux extraction with xylene. The absence of significant gel formation shows that the material has been modified without any crosslinking of the elastomer to improve the melt strength. The use of peroxide at this high concentration, however, has been found to cause detrimental deterioration of the molecular weight of the polypropylenic polymer.
U.S. Pat. No. 5,569,717 and Graebling et al., Journal of Applied Polymer Science, Vol 66, pp. 809-819, 1997, disclose that a multifunctional co-agent, or monomer, can be used to modify the rheology of polypropylene-containing materials via peroxide initiation. The preferred compositions contain 10 to 25 weight percent polyethylene with a density greater than 0.92 g/cm3, more than 0.5 weight percent of trimethylolpropane triacrylate (TMPTA), and between 0.01 and 0.1 weight percent organic peroxide. These materials exhibit greatly improved melt strength for extrusion processing and thermoforming, but the resultant compositions are hard and rigid at room temperature and can therefore not be used as an alternative to plasticized polyvinyl chloride. The importance of the polyethylene for improved melt strength is demonstrated by the examples described in U.S. Pat. No. 5,569,717. The polyethylene used therein, however, was Solvay ELTEX(copyright) A1050, a high rigidity material with a density of 0.961 g/cm3.
U.S. Pat. No. 6,207,746 discloses a process for producing thermoplastic elastomers with olefin-elastomer and polypropylene via a radical-initiated mechanism. The patent further teaches that radical initiators above a concentration of 0.02 parts by weight of 100 parts by weight of the elastomer are required to accomplish a sufficient degree of crosslinking and that both tri-methacrylate and tri-acrylate co-agent monomers are useful to increase the crosslinking efficiency.
Thus, there is a need for soft plastic materials for fabrication of fully recyclable articles via processes that require high melt strength.
The present invention successfully improves the Theological properties in the molten state for each component in an olefinic thermoplastic elastomer (TPE) blend. The modified olefinic TPE exhibits an increased resistance to deformation during elongation or extension and does not exhibit the disadvantages of the prior art compositions.
The invention relates to a thermoplastic elastomer composition comprising a modified blend of a propylenic resin, an ethylenic elastomer, and a multifunctional acrylic monomer comprising at least three acrylate groups, or a reaction product thereof, with the ethylenic elastomer being present in an amount by weight that is greater than that of the propylenic resin and wherein (a) the propylenic resin is at least partially branched, (b) the ethylenic elastomer is at least partially crosslinked to a gel content of at least about 25%, or (a) and (b), the modified blend having a ratio of the melt strength of the modified blend to the melt strength of an unmodified blend of a propylenic resin that is not branched and an ethylenic elastomer that is not crosslinked of about 1.5 to 15 measured at a temperature of at least about 180xc2x0 C., a melt flow rate of less than about 1 dg/min measured at 230xc2x0 C. under a load 2.16 kg, a melt flow rate of less than about 5 dg/min measured at 230xc2x0 C. under a 10 kg load, and a hardness of less than about 95 Shore A or less than about 45 Shore D.
In preferred embodiment, the ethylenic elastomer is at least partially crosslinked. The ratio of the melt strength of the modified blend to the melt strength of the blend before modification can be about 1.6 to 12 measured at a temperature of at least about 180xc2x0 C. In one embodiment, the reaction of the propylenic resin, the ethylenic elastomer, and the multifunctional acrylic monomer is initiated by heat activation at a temperature of about 200xc2x0 C. to 250xc2x0 C. In another embodiment, the reaction of the propylenic resin, the ethylenic elastomer, and the multifunctional acrylic monomer is initiated by the addition of less than about 0.3 pph of a free radical initiator to form the modified blend. In a embodiment, the free radical initiator has a decomposition half-life of greater than about one hour at 120xc2x0 C.
In one embodiment, the modified blend includes about 5 weight percent to up to less than 50 weight percent propylenic resin and greater than 50 weight percent to about 95 weight percent of the ethylenic elastomer. In a preferred embodiment, the modified blend includes about 15 weight percent to 48 weight percent propylenic resin and about 52 weight percent to 85 weight percent of the ethylenic monomer. In another embodiment, the propylenic resin includes at least about 60 mole percent propylene monomer and the ethylenic elastomer includes at least 60 mole percent ethylene monomer.
In one embodiment, the ethylenic elastomer has a Mooney viscosity of at least about 15, a molecular weight of greater than about 80,000, and a polydispersity of greater than about 1.5. In yet another embodiment, the ethylenic elastomer has a density of less than 0.94 g/cm3. In one embodiment, the multifunctional acrylic monomer is present in an amount of about 0.1 pph to 5 pph of the polymers and has no more than seven acrylate groups.
In preferred embodiment, the multifunctional acrylic monomer includes trimethylolpropane triacrylate (TMPTA), ethoxylated trimethylolpropane triacrylate, propoxylated glycerol triacrylate, pentaerythritol triacrylate, bistrimethylolpropane tetraacrylate, pentaerythritol tetraacrylate, ethoxylated pentaerythritol tetraacrylate, trimethylolpropane trimethacrylate, ethoxylated pentaerythritol triacrylate, or combinations thereof. In any of the embodiments, the propylenic resin can include a homopolymer of propylene and a copolymer of propylene and at least one monomer including C2 to C20 alpha-olefins, unsaturated organic acids and their derivatives, vinyl esters, aromatic vinyl compounds, vinylsilanes and unconjugated aliphatic and monocyclic diolefins, alicyclic diolefins which have an endocyclic bridge, conjugated aliphatic diolefins, and combinations thereof; and the ethylenic elastomer can include a copolymer of ethylene and at least one monomer comprising C3 to C20 alpha-olefins, unsaturated organic acids and their derivatives, vinyl esters, aromatic vinyl compounds, vinylsilanes and unconjugated aliphatic and monocyclic diolefins, alicyclic diolefins which have an endocyclic bridge and conjugated aliphatic diolefins, or terpolymers of at least 60 mole percent of ethylene, a C3 to C20 alpha-olefin, a nonconjugated diene monomer, or combinations thereof.
The blends of the invention can also include one or more thermal stabilizers, ultraviolet stabilizers, flame retardants, mineral fillers, extender or process oils, conductive fillers, nucleating agents, plasticizers, impact modifiers, colorants, mold release agents, lubricants, antistatic agents, pigments, and the like.
The invention also relates to compositions prepared by the process of melt blending the propylenic resin, the ethylenic elastomer, and the multifunctional acrylic monomer specified above, preferably while initiating the reaction thereof with either heat activation, a free radical initiator, or both. Further, the invention relates to articles including the composition of the invention described above, which is formed by extrusion, thermoforming, blow molding, foam processing, or calendering. In a preferred embodiment, the article is in the form of an automobile component.
The invention relates to a method for preparing a polymer blend including combining a propylenic resin, an ethylenic elastomer that is at least partially crosslinked, and a multifunctional acrylic monomer in the presence of an optional free radical initiator, to form a polymer mixture, melt blending the polymer mixture at a temperature above the melt point of the propylenic resin and below about 180xc2x0 C. for about 5 to 20 seconds, and mixing the polymer mixture at a temperature of about 160xc2x0 C. to 250xc2x0 C. for at least about 10 to 100 seconds to at least partially crosslink the ethylenic elastomer, thereby providing a modified polymer blend having a ratio of the melt strength of the modified blend to the melt strength of an unmodified blend of a propylenic resin that is not branched and an ethylenic elastomer that is not crosslinked of about 1.5 to 15 measured at a temperature of at least about 180xc2x0 C., a melt flow rate of less than about 1 dg/min measured at 230xc2x0 C. under a 2.16 kg load, a melt flow rate of less than about 5 dg/min measured at 230xc2x0 C. under a 10 kg load, and a hardness of less than about 95 Shore A or less than about 45 Shore D.
The invention also relates to a thermoplastic elastomer composition including a modified blend of a propylenic resin, a styrenic elastomer, and a multifunctional acrylic monomer comprising at least three acrylate groups, or a reaction product thereof, wherein (a) the propylenic resin is at least partially branched, (b) the styrenic elastomer is at least partially crosslinked to a gel content of at least about 25%, or (a) and (b), the modified blend having a ratio of the melt strength of the modified blend to the melt strength of an unmodified blend of a propylenic resin that is not branched and a styrenic elastomer that is not crosslinked of about 1.5 to 15 measured at a temperature of at least about 180xc2x0 C., a melt flow rate of less than about 1 dg/min measured at 230xc2x0 C. under a 2.16 kg load, a melt flow rate of less than about 5 dg/min measured at 230xc2x0 C. under a 10 kg load, and a hardness of less than about 95 Shore A or less than about 45 Shore D.
The invention further relates to a method for preparing a composition by combining a propylenic resin that is at least partially branched, an ethylenic elastomer, and a multifunctional acrylic monomer, to form a polymer mixture, melt blending the polymer mixture at a temperature above the melt point of the propylenic resin and below about 180xc2x0 C. for about 5 to 20 seconds, and mixing the polymer mixture at a temperature of about 160xc2x0 C. to 250xc2x0 C. for at least about 10 to 100 seconds to provide a modified polymer blend having a ratio of the melt strength of the modified blend to the melt strength of an unmodified blend of a propylenic resin that is not branched and an ethylenic elastomer that is not crosslinked of about 1.5 to 15 measured at a temperature of at least about 180xc2x0 C., a melt flow rate of the modified blend of less than about 1 dg/min measured at 230xc2x0 C. under a 2.16 kg load, a melt flow rate of the modified blend of less than about 5 dg/min measured at 230xc2x0 C. under a 10 kg load, and a hardness of less than about 95 Shore A or less than about 45 Shore D.
The invention also relates to embodiments above where a styrenic elastomer is at least partially or even entirely subsituted for the ethylenic elastomer. In one preferred embodiment, styrene forms at least about 50 mole percent of the styrenic elastomer portion of the blend. Any suitable styrenic elastomer or combination thereof can be included in forming the modified blend, including styrene in copolymers with various monomers. For example, styrene-butadiene, styrene-ethylene-butylene-styrene, or the like can be included.